How to Migrate CI Incrementally by Running Both in Parallel

A safe cutover runs the old CI and GitHub Actions side by side, keeps the old one authoritative, and only makes Actions a required check once results match.

Do not flip everything at once. Add the Actions workflow alongside the existing pipeline, leave the old system as the required check, compare outcomes for a while, then swap the required status to Actions and retire the old config.

Phases

Shadow workflow

# Runs in parallel with the old CI but does not block merges yet.
on: [push, pull_request]
jobs:
  ci:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - run: npm ci && npm test

What does not map cleanly

• Running two systems doubles minutes during the overlap; keep the shadow window short, or use cheaper managed runners such as Latchkey while you compare.
• Only one system should be a required check at a time to avoid confusing merge gates.
• Remove the old config in the same PR that promotes Actions so history stays clear.

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